It's been a significant embedded OS since the late 1980s, with many millions of deployed systems.
It is also used in roles with a customer-facing UI and indeed GUI. It's in several car media players, for instance, including from Toshiba I believe.
And of course it was the basis of Blackberry 10.
https://crackberry.com/blackberry-10
It was in the Playbook, the cancelled Folio, and a lot of smartphones, such as the Passport I used to own. (A lovely device, sadly crippled as messaging app vendors removed support. I could use Android apps on it, but they didn't integrate into the Blackberry Hub.)
It was also very nearly the next-gen Amiga, which I suspect is why it has a GUI and multimedia support.
https://www.trollaxor.com/2005/06/how-qnx-failed-amiga.html
I suspect that between routers, switches, engine control units, traffic signals, smartphones, car entertainment units, and doubtless many other things we don't hear about, it's in more devices in production than all of seL4, Fiasco, etc. put together.
Minix 3 is arguably also a true microkernel, and it's present in industry, embedded in millions of Intel Xeon and Core iX CPUs' management units, but it never reached a state of much completeness. No SMP support, for instance, and a lot of missing functionality.
And it now seems to be dead: